What do glacial surfaces and tea have in common? The obvious answer to a glaciologist… ‘a glaciologist – standing on the surface of a glacier, drinking tea’. While this is, of course, correct, here’s another that is more interesting and of high relevance to the Dark Snow Project; the surface’s colour. Or – more precisely – its pigmentation.
The Dark Snow Project is interested in all things dark on the surface of glaciers and ice sheets. Joe Cook summarizes the ways microbes can contribute to the darkening of glacial surfaces. I’ll focus on a specific process that may be the key biotic factor for albedo reduction of the Greenland ice sheet.
There are three species of algae that grow on glacier surfaces worldwide – Cylindrocystis brébissonii, Mesotaenium berggrenii, and Ancylonema nordenskiöldii. I’ll call them ‘ice algae’. Each belongs in a single group of green algae called Zygnematophyceae, also called conjugating algae due to their inventive and esthetic way of having sex. It is still a mystery why only these few species reside on ice, and from only one group, but we begin to understand how: it’s to do with tea!
Imagine the environment of the surface of the Greenland ice sheet… It’s of course cold, around freezing all the time, and it can be very bright, further amplified by ice crystals reflecting the incoming light in all directions and multiple bounces between clouds and the surface increases the glare. If you’re an alga, then you too are cold and you’re dazzled much of the time. You get most of your food from the atmosphere as carbon dioxide, but you still need some other nutrients such as nitrogen and phosphorus, and there is not much of that around. And you may even need to protect yourself from large predators that stalk these vast white planes. Ok, not large for humans. Microscopical predators. But still scary.
It seems that ice algae have solved these problems in one fell swoop by producing a special pigment stored small vesicles (called vacuoles) inside the cells, and its colour has been described (see Yallop et al. 2012). Now, depending on the observers’ state of colour-blindness and/or gender, the pigment is seen as purple brown (Yallop et al. 2012), brownish (Remias et al. 2012), and dark brown (Uetake et al. 2010). I am going to call it ‘dark’. Until recently, the chemical nature of this dark pigment was unknown. But then Daniel Remias and his colleagues from Innsbruck University decided to look at one of the ice algae – M. berggrenii from an Alpine glacier – more closely (Remias et al. 2012). They managed to resolve the structure of the main compound responsible for the dark colour and gave it a beautiful name: purpurogallin carboxylic acid-6-O-b-D-glucopyranoside.
What can purpurogallin carboxylic acid-6-O-b-D-glucopyranoside do? Well, it’s dark, so of course it absorbs sunlight, mostly in the ultraviolet and visible parts of the spectrum. In other words, it is a sunscreen, a protection against the harmful UV radiation and also excessive visible radiation which can inhibit photosynthesis in the cells. But that’s not all. The pigment may also represent a sink for surplus energy that cannot be invested in cells due to limitations in temperature or nutrient availability, and may even act as a chemical defense against grazers as, for example, phenolic compounds in marine kelp. So, given the nuisances you have to put up with as an alga living on the surface of an ice sheet, it seems like a very useful thing to have.
And strangely, purpurogallin carboxylic acid-6-O-b-D-glucopyranoside is a pigment that has only been detected in higher plants, such as fermented plant tissue… leaves, more specifically… fermented leaves of Camellia sinensis, even more specifically. Also known as black tea.
So here we are, glaciologists standing on the melting surface of the Greenland ice sheet, sipping tea that is black precisely for the same reason why the ice surface is getting dark – a simple pigment produced by a living organism.
We promise we won’t spill much of the tea.
Uetake J, Naganuma T, Hebsgaard MB, Kanda H, Kohshima S (2010) Communities of algae and cyanobacteria on glaciers in west Greenland. Polar Science 4: 71–80
Remias D, Schwaiger S, Aigner S, Leya T, Stuppner H, Lütz C (2012) Characterization of an UV- and VIS-absorbing, purpurogallin-derived secondary pigment new to algae and highly abundant in Mesotaenium berggrenii (Zygnematophyceae, Chlorophyta), an extremophyte living on glaciers. FEMS Microbiology Ecology 79: 638–648
Yallop ML, Anesio AM, Perkins RG, Cook J, Telling J, Fagan D, MacFarlane J, Stibal M, Barker G, Bellas C, Hodson A, Tranter M, Wadham J, Roberts N (2012) Photophysiology and albedo-changing potential of the ice algae community on the surface of Greenland Ice Sheet. ISME Journal 6: 2302–2313